Fixes zombies missing claws on transformation (#5998)

# About the pull request
Fixes #5485, added a simple conditional to check if a mob is buckled and
then unbuckle them before transformation. I didn't look into the root of
the problem but this bug seems niche enough that it shouldn't really
effect anything else. If I were to guess, I'm assuming it has to do with
the fact that you can't hold items in your hands while buckled to a bed,
but you can while you're in a chair. Hence why everything works fine
while buckled to a chair.

# Changelog
:cl:
fix: Fixes zombies missing claws on transformation
/:cl:

code/datums/diseases/black_goo.dm

@@ -148,6 +148,8 @@
 	human.make_jittery(500)
 	sleep(30)
 	if(human && human.loc)
+		if(human.buckled)
+			human.buckled.unbuckle()
 		if(human.stat == DEAD)
 			human.revive(TRUE)
 			human.remove_language(LANGUAGE_ENGLISH) // You lose the ability to understand english. Language processing is handled in the mind not the body.